Recently, researches on hybrid type construction machinery, which improves fuel efficiency by storing surplus power of an engine in a battery, and supplying power from the battery to the engine that does not have sufficient power so as to cope with an rapid increase in oil price, are being actively conducted.
A system, which uses the engine and an electric motor as a common power source as described above, and has an electrical energy storage device, is referred to as a hybrid system. For example, as the hybrid system, there is a hybrid system for heavy equipment, such as a hybrid vehicle and an excavator.
In the meantime, a general excavator system uses an engine as a power source and serves to swing or move a boom, an arm, and a bucket, which are final loads, by means of hydraulic pressure. On the contrary, in the case of a hybrid excavator system, a motor and an electricity storage device are additionally installed in a general excavator, thereby improving overall efficiency of the excavator system.
FIG. 1 is a configuration diagram of a hybrid excavator, in which a swing apparatus is driven by electricity by being separated from a hydraulic power train system in the related art.
As illustrated in FIG. 1, a swing motor 70 is driven by receiving electric energy from an energy storage device 60, and the energy storage device 60 receives electric power from an engine auxiliary motor 20. In a situation where the swing motor 70 decelerates, kinetic energy of the swing apparatus is regenerated to the energy storage device 60, and regarding electric power that is lost due to friction and the like, the engine auxiliary motor 20 supplies electric power to maintain a voltage of the energy storage device 60 to maintain at an appropriate level. In the meantime, an engine 10, a pump 30, a control valve 40, and a boom/arm/bucket traveling operator 50 are the same components as those provided in an engine type excavator in the related art.
The engine of the excavator in the related art is controlled so as to receive a target engine rotational speed from a dial input of a driver and a mode input switch (for example, a power (P) mode, a standard (S) mode, and an economy (E) mode) as illustrated in FIG. 2 and maintain the received target engine rotational speed with a predetermined uniform value. Accordingly, the engine is driven at a predetermined rotational speed regardless of a load size, so that unnecessary energy loss is generated in low load work.
In the meantime, in the hybrid excavator in which the swing apparatus is electrically driven by being separated from the hydraulic power train system in the related art as illustrated in FIG. 1, the swing motor 70 receives electric energy from the energy storage device 60 to be driven. In the aforementioned hybrid excavator, a load is not generated in the engine during the turning operation, so that, in a representative work mode, in which excavation-turning-dump operations are repeated, a time, for which a high load is applied to the engine, is shorter than that of the engine type excavator in the related art. Accordingly, when a method of controlling a rotational speed of an engine in the related art is applied, the hybrid excavator has larger energy loss due to an unnecessarily high rotational speed of the engine at low load work, compared to the engine type excavator in the related art.
However, in the hybrid excavator of FIG. 1, in order to maintain a voltage of the energy storage device 60 at an appropriate level, the engine auxiliary motor 20 driven at the same rotational speed of that of the engine 10 needs to generate a sufficient power generation voltage while rotating at an appropriate speed or more. This means that in the hybrid excavator, a rotational speed of the engine needs to be maintained high in order to charge the energy storage device 60 even under low load work, differently from the engine type excavator in the related art.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.